A volume holographic recording system is known as a digital information recording system using the principle of hologram. The feature of this system is to record an information signal in a recording medium as variations in a refractive index. In the system, photorefractive material such as lithium niobate single crystal and the like is used for the recording medium.
As one of hologram recording and reproducing methods, there is a method for recording and reproducing by use of Fourier transform.
FIG. 1 shows an example of a conventional hologram recording and reproducing apparatus. In this drawing, laser light 12 emitted from a laser light source 11 is divided into a signal light 12A and a recording reference light 12B by a beam splitter 13. The beam diameter of the signal light 12A is magnified by a beam expander 14, and the signal light 12A is applied to a spatial light modulator (SLM) 15 such as a panel of a translucent TFT liquid crystal display (LCD) and the like as collimated light. The spatial light modulator (SLM) 15 receives recording data converted by an encoder 25 as an electric signal, to form a bright and dark dot pattern on a plane. In passing through the spatial light modulator (SLM) 15, the signal light 12A is modulated to include a data signal component. When the signal light 12A including the signal component of the dot pattern passes through a Fourier transform lens 16, which is disposed a focal length “f” away, the signal component of the dot pattern is subjected to Fourier transform, and is condensed into the recording medium 5.
On the other hand, the recording reference light 12B divided by the beam splitter 13 is led into the recording medium (volume holographic memory) 5 by a mirror 18 and a mirror 19. The recording reference light 12B intersects with an optical path of the signal light 12A inside the recording medium 5 and forms a light-interference pattern, to record the whole light-interference pattern as variations in a refractive index.
The Fourier transform lens, as described above, forms an image from diffracted light of image data, which is illuminated by coherent collimated light. The image is converted into distribution on a focal plane, that is, on a Fourier plane, and the distribution as a result of Fourier transform is allowed to interfere with the coherent reference light, in order to record interference fringes on the recording medium in the vicinity of a focal point. After completing the recording of a single data page (hereinafter, also simply referred to as a “page”), the mirror 19 is rotated at a predetermined angle, and the position thereof is moved in parallel by a predetermined amount, in order to vary an incident angle of the recording reference light 12B with respect to the recording medium 5. Then, the second page is recorded in the same procedure. Angular multiplexing recording is carried out by successively performing the recording like this.
In reproducing operation, on the other hand, inverse Fourier transform is carried out to reproduce a dot pattern image. In reproducing data, as shown in FIG. 1, the optical path of the signal light 12A is interrupted by, for example, the spatial light modulator (SLM) 15, and only the reference light 12B is applied to the recording medium 5. During reproduction, the position and angle of the mirror 19 are varied and controlled with the use of the combination of the rotation and linear movement of the mirror 19 so that the incident angle of the recording reference light becomes the same as that in recording a page to be reproduced. Reproduction light which reproduces the recorded light-interference pattern appears on the opposite side of the recording medium 5 irradiated with the reference light 12B. A dot pattern signal can be reproduced by leading the reproduction light into an inverse Fourier transform lens 16A to carry out inverse Fourier transform. Then, the dot pattern signal is received by a photodetector 20 such as a charge-coupled device CCD and the like in the position of a focal length, to reconvert the dot pattern signal into an electric digital data signal. Then, the digital data signal is sent to a decoder 26, so that original data is reproduced.
In the hologram recording with Fourier transform, the first-order diffracted light becomes the highest frequency component of the signal light, which is Fourier transformed by the spatial light modulator 15 such as the LCD and the like, due to the repeats of pixels of the spatial light modulator 15.
FIG. 2 is a plan view showing a pattern of the conventional spatial light modulator 15. Square pixels a single side of which has a length of “a” (μm) are arranged in a matrix. In other words, a pixel pitch of the spatial light modulator 15 is “a” (μm). The reference numeral 6 indicates an incident beam which is incident on the spatial light modulator 15.
Referring to FIG. 3, the optical axis of the signal light represents a Z direction, and the directions of columns and rows of the pixels in a plane perpendicular to the signal light represent X and Y directions, respectively. When the signal light interferes with the reference light to record inside the recording medium 5, light intensity distributions of spatial frequency spectrum occur in the XY plane, which is in parallel with the Fourier plane, symmetrically with respect to the optical axis of the signal light.
The hologram recording using a Fourier transform hologram has the advantages that hologram fits into spatially limited space, information is recorded in a distributed manner by use of Fourier transform, and the redundancy of recording can be increased. The distance (d1) between a zero-order Fourier spectrum and the first-order Fourier spectrum in the Fourier plane is expressed as follows, with the use of a spatial frequency (fsp) in a recording plane, the wavelength (λ) of light, and the focal length (F1) of the Fourier transform lens.d1=fsp•λ•F1
Since the pixel pitch of the spatial light modulator 15 is 42 μm, the wavelength is 532 nm, and the focal length is 165 mm, the Fourier spectrum distance (d1) of the corresponding highest frequency component is 2.1 mm, according to the foregoing equation. Thus, information to be recorded exists in a range of approximately ±2.1 mm on the optical axis. In other words, as shown in FIG. 3, two-dimensional data appearing in the spatial light modulator 15 is distributed over xy space (x, y≦±2d1) in a matrix with two rows and two columns, which is composed of the first-order diffracted light and zero-order light.
Therefore, a peak appears in a Fourier transformed image of the spatial light modulator 15, in accordance with the highest frequency component due to the pixel pitch. These peaks themselves do not bear any meaningful data. If these peaks occur in such a Fourier transformed image, the photorefractive effect of the recording medium becomes saturated in the above-mentioned peak position, so that there is a problem that nonlinear distortion tends to occur in a recorded image.
Also there is a method for offsetting the recording medium from the Fourier plane in order to secure a dynamic range during recording, but the method has the problems that time necessary for recording becomes long, an S/N ratio decreases, highly sensitive recording medium is needed, and the like.
Considering the foregoing problems, an object to be achieved by the present invention includes one example of the foregoing problems. In other words, an object of the present invention is to provide a spatial light modulator with high performance which can record with high sensitivity and less signal distortion.